Since the advent of vertical transistors in trench-type dynamic random access memories (DRAMs), those skilled in the art will recognize that the actual gates of such devices are formed by a polysilicon plug inside an upper portion of a trench, the lower portion containing a storage capacitor. However, the wordlines which access the vertical transistor gates extend in linear paths above the substrate, and have a similar structure and extent to actual gate conductors, which contact the gate oxide of planar transistors. Such wordlines, and actual gate conductors are formed as line structures in a front-end-of-line (FEOL) process. Both types often have a stacked structure, including a lower layer of polysilicon, and an overlying low resistance layer, often including a metal silicide, over which a capping layer may optionally be formed. While they can both be referred to as FEOL conductive line stacks, in the description which follows, the terms “gate” and “gate conductor” are intended to refer to either type of structure, whether the “gate conductor” actually and directly contacts a gate oxide of a transistor, or whether it merely acts as an FEOL line conductor, for example as a wordline for accessing a vertical transistor of a trench DRAM, or for some other purpose as an FEOL conductor.
In the manufacture of integrated circuits, including DRAMs, the manufacturing of gate conductors has become more and more important. Particularly, in dynamic random access memories (DRAMs), gate conductors must be manufactured at very tight pitches, requiring gate height to be limited to allow effective insulative gapfill between adjacent gate conductors. On the other hand, the lengths and narrowness required of gate conductors demands that resistance along the gate conductor be kept within a tolerable limit.
The use of tungsten (W) as the gate conductor material is receiving much interest today. While polysilicon, tungsten silicide and/or a combination of the two have been popular as gate conductor materials up to now, smaller groundrules and faster speeds required in new generations demand a lower resistance gate conductor. A low resistance gate conductor material is needed for both speed and to keep the height of the gate stack low enough to permit gaps between them to be filled with an insulator. Accordingly, there is great interest today in using tungsten instead of tungsten suicide as the gate conductor.
However, the use of tungsten creates a new set of challenges. For one, in a gate stack of polysilicon and tungsten, tungsten tends to be oxidized during the selective oxidation of the polysilicon gate sidewall and/or during the deposition of gate silicon nitride spacers. Sidewall oxidation of the polysilicon is necessary to heal damage from the gate stack etch, and to round gate polysilicon corners, which could otherwise give rise to corner conduction and gate oxide breakdown. However, the poly sidewall oxidation can seriously deteriorate the tungsten. Tungsten is unlike some other metals in that when it Is oxidized, a spike-like WOx “grass” is formed which “grows” out in many directions, and which may even extend far into the material. Once the tungsten oxide extends internally in such manner, efforts to remove the oxide from the material are ineffective. Consequently, a need exists for engineering the tungsten surface to prevent or reduce oxidation of tungsten when performing the gate polysilicon sidewall oxidation.
Apart from the poly sidewall oxidation, other process steps also tend to form tungsten oxides during gate stack processing, including gate sidewall spacer formation, and even oxidation just by moving the substrate from one process tool to another, because of oxygen being present in the ambient. Hence, a tightly controlled process would have to be used to remove oxygen from the ambient, such as maintaining a vacuum In the chamber, and/or pumping N2 into the chamber, and doing the same for airlock and “loadlock” chambers between successive process chambers.